<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            margin: 0;
            padding: 0;
        }
        .box{
            display: flex;
        }
        .left,.right{
            width: 300px;
            height: 100vh;
            background-color: gray;
        }
        .middle{
            flex: 1;
            background-color: lightcyan;
        }
        </style>
</head>
<body>
    <div class="box">
        <div class="left">
            左边固定宽度
        </div>
        <div class="middle">Content</div>
        <div class="right">
            右边固定宽度
        </div>
</body>
</html>